1. Field of the Invention
The present invention relates to a fixing device built in an electrophotographic image forming apparatus, such as a laser printer, a laser facsimile, and a digital copier, and an image forming apparatus having the fixing device.
2. Description of the Related Art
Generally, in electrophotographic image forming apparatuses, a photosensitive drum is almost uniformly charged. The photosensitive drum is exposed to a laser scanning unit (LSU) or the like to form an electrostatic latent image thereon corresponding to a desired image signal. Toner charged by a developing device is supplied to the photosensitive drum to form a toner image. The toner image is transferred to a recording medium. The toner image transferred to the recording medium is carried on the recording medium, but not fixed to the recording medium. Accordingly, by heating and pressing the toner image with a fixing device built in the image forming apparatus, the toner image is thermally fused and fixed to form a fixed image on the recording medium.
A conventional fixing device basically includes a cylinder-shaped heating roller (that is, a heating member) made of metal, a pressing roller (that is, a pressing member) compressed onto the heating roller, and a halogen lamp (that is, a heat source) that is inserted into the heating roller to heat the heating roller with radiant heat. In such a roller type fixing device, a recording medium carrying the toner image is conveyed to a nip section in which the heating roller and the pressing roller are mutually compressed, thereby heating the toner image on the recording medium by means of the heat of the heating roller and pressing by means of the compression of the heating roller and the pressing roller to fix the toner image onto the recording medium.
The conventional fixing device requires much time to heat a room temperature heating roller to a fixing temperature of 160 to 200° C. at which the toner is fused. Accordingly, the conventional fixing device requires considerable warm-up time to practically start a print job after it is powered on. Additionally, the fixing device should be always kept at a high temperature to shorten the time from standby status to the start of a print job. As a result, since the halogen lamp should be always turned on to keep the fixing device at a high temperature, the power consumption is increased.
Various fixing devices capable of shortening the warm-up time and reducing the power consumption have been suggested to solve the problems associated with conventional fixing devices.
For example, an induction heating type fixing device induces current with a magnetic flux to generate heat. The induction heating type fixing device basically includes a cylinder-shaped or semi-circular sectional holder, a film that is extrapolated onto the holder and that has a conductive layer and a resistor layer stacked on each other. A magnetic field generating unit has an exciting coil and a magnetic core provided inside the holder. A pressing roller is compressed onto the outer circumferential surface of the film to form a fixing nip section. In the induction heating type fixing device, eddy current is generated by means of the magnetic flux generated by the magnetic field generating unit, Joule heat is generated in the conductive layer by means of the eddy current, and the temperature of the fixing nip section is increased by means of the Joule heat. Thus, the temperature of the nip section is increased to the fixing temperature in a short period of time, thereby shortening the warm-up time. Additionally, since the film has a thermal capacity lower than that of the heating roller, the power consumption is reduced. Japanese Unexamined Patent Application Publication No. 2004-126274 discloses a conventional induction heating type fixing device. However, since the magnetic field generating unit should be provided, there is a problem that the component cost is greater than that of the roller-type fixing device. Additionally, since the inner circumferential surface of the film slides with respect to the holder or the like, the life-time of the film is limited by means of wear of the sliding portions and the range for selection of materials is narrowed.
Furthermore, a pressing belt type fixing device has a pressing roller of the roller-type fixing device replaced with a pressing belt of a tube shape and a part of the pressing belt is compressed to the heating roller from the inside thereof to form a fixing nip section. The pressing belt type fixing device basically includes a heating roller that is heated by a heat source, such as a halogen lamp, and that pivotally rotates. A pressing belt abuts the heating roller and rotates along with the heating roller. A pressing member presses the pressing belt from the inside thereof to the heating roller. In the pressing belt type fixing device having the above-mentioned structure, the pressing belt is compressed to the heating roller to form the fixing nip section, whereby the nip section is wide. Accordingly, it is possible to shorten the warm-up time and also to reduce the power consumption. Additionally, the component cost is reduced lower than that of the induction type fixing device. Japanese Unexamined Patent Application Publication No. 2004-12682 discloses a pressing belt type fixing device. However, since the inner circumferential surface of the pressing belt is pressed by the surface of the pressing member and slides, the life-time of the pressing belt or the pressing member is limited by means of wear of the sliding portion and the range for selection of materials is narrowed.
Furthermore, a heating belt type fixing device has a heating roller of the roller-type fixing device replaced with a heating belt unit having a heating belt circumferentially movable and the heating belt and the pressing roller are compressed to each other to form a fixing nip section. The heating belt type fixing device basically includes a heating belt unit in which a heating belt is wound around a support roller and a cylinder-shaped heating roller. A cylinder-shape pressing roller is compressed onto a fitting portion of the heating belt between the support roller and the heating roller. A halogen lamp is built in the heating roller or the pressing roller. In the heating belt type fixing device having the above-mentioned structure, since the fixing nip section is wide, the warm-up time is shortened and the power consumption is reduced. Additionally, since no sliding portion exists, no member is worn away and the range for selection of materials is widened. It is also possible to reduce the component cost lower than that of the fixing device of an induction heating type. Japanese Unexamined Patent Application Publication No. 2004-205877 discloses a heating belt type fixing device.
However, in the conventional heating belt type fixing device, the heating belt is made to extend by the support roller and the heating roller and tension is applied to the heating belt. As a result, by circulating the heating belt, the heating belt is moved in the axial direction of the support roller or the heating roller and is deviated toward one side due to error in measurement accuracy or assembly accuracy of the respective rollers or widthwise deviation in circumferential length of the heating belt.
Additionally, in the conventional heating belt type fixing device, the temperature of the heated fixing nip section becomes uniform as a whole. As a result, when the toner image carried on the recording medium passes through the entrance of the fixing nip portion, the toner image may not be sufficiently heated. Therefore, the interface between the toner and the recording medium may not be sufficiently fused, whereby there may be a phenomenon (cold off-set) in which a part of the toner image is attached to the heating belt by means of an adhesive force or an electrostatic force between the toner and the heating belt. Furthermore, when the toner image carried on the recording medium passes through the exit of the fixing nip portion, the toner image may be heated too much. As a result, a cohesive force of the toner is smaller than the adhesive force between the belt and the recording medium, the toner image may be separated and there may occur a phenomenon (hot off-set) in which a part of the separated toner image is attached to the belt.
Accordingly, a need exists for an improved fixing device for an image forming apparatus in which the warm-up time is shortened, power consumption is reduced and deviation of a belt is reduced.